Crankcase integrity breach detection

ABSTRACT

Methods and systems are provided for using a crankcase vent tube pressure or flow sensor for diagnosing a location and nature of crankcase system integrity breach. The same sensor can also be used for diagnosing air filter plugging and PCV valve degradation. Use of an existing sensor to diagnose multiple engine components provides cost reduction and sensor compaction benefits.

BACKGROUND/SUMMARY

Engines may include an air filer in the air intake passage for filteringair entering a turbocharger compressor (if the engine is boosted) or anintake throttle (if the engine is naturally aspirated). Air filters maybecome plugged over time leading to pressure drops across the filter.Accordingly, air filters may be intermittently diagnosed so that avehicle operator can be informed about changing his filter.

Example approaches used to monitor air filter plugging involve the useof manifold air flow sensors to detect changes in air flow resultingfrom the plugging, the use of dedicated pressure sensors to detectpressure depression downstream if the air filter, etc. In response tofilter plugging, a controller may limit engine power to reducecompressor over-speeding.

However, the inventors herein have recognized potential issues with suchapproaches. As one example, MAF sensor data may be less desirable thanspeed density data for engine control, making the MAF sensors lessavailable. As another example, dedicated pressure sensors positioneddownstream of the filter may be effective but can add substantial costand complexity to the system.

In one approach, to at least partially address these issues, a methodfor an engine is provided. The method comprises, indicating filterdegradation based on a pressure sensor in a crankcase vent tube. In thisway, an existing pressure sensor can be advantageously used to identifyair filter plugging.

In one example, an engine crankcase ventilation system may include acrankcase vent tube coupled between an air intake passage and acrankcase. A pressure sensor (or flow sensor) may be positioned withinthe crankcase vent tube for providing an estimate of flow or pressure ofair flowing through the vent tube. During conditions when manifold airflow is lower than a threshold flow, such as during engine cranking, anengine controller may learn an offset and a reference pressure for thepressure sensor. For example, the pressure sensor may be a firstpressure sensor, and based on whether the pressure sensor is an absolutepressure sensor or a gauge pressure sensor, the controller may comparethe output of the first pressure sensor to a barometric pressure (BP)reading estimated by a second pressure sensor, and learn a sensoroffset. Then, during conditions when manifold air flow is higher thanthe threshold flow, such as after engine cranking when an engine speedis sufficiently high, the controller may adjust the output of the firstvent tube pressure sensor based on the learned offset, and determine airfilter plugging based on a deviation of the adjusted output from thereference pressure estimated during low air flow conditions. Forexample, based on the adjusted output of the first vent tube pressuresensor deviation from an inferred BP estimate, air filter plugging maybe determined. In response to the air filter plugging, a mitigatingaction may be performed. For example, a diagnostic code may be set andengine speed may be limited to reduce compressor damage fromover-speeding or over-heating.

In this way, by using an existing crankcase ventilation system pressuresensor to identify air filter plugging, the need for additional sensorsand valves for monitoring air filter degradation is reduced, providingcost and complexity reduction benefits without reducing the accuracy ofdegradation detection. By learning offsets in the pressure sensor duringlow engine air flow conditions and then applying the learned offsetsduring high engine air flow conditions, crankcase vent tube pressure maybe accurately and reliably used to indicate air filter plugging. Byrelying on vent tube pressure, rather than MAF, to diagnose an airfilter, the need for MAF sensor data in diagnosing air filter pluggingis reduced. Further, the approach enables the crankcase ventilationsystem to remain active during a diagnostic procedure.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial engine view in accordance with the disclosure.

FIGS. 2A-B show a high level flow chart for indicating degradation ofone or more crankcase ventilation system components based on changes incrankcase vent tube pressure during cranking and/or engine running.

FIGS. 3-4 show example methods for indicating crankcase ventilationsystem breach, as well as a location of crankcase ventilation systembreach, based on a transient dip in crankcase vent tube pressure duringengine cranking and changes in crankcase vent tube pressure relative tochanges in manifold air flow during engine running.

FIG. 5 shows an example method for indicating PCV valve degradationbased on changes in crankcase vent tube air flow during condition of lowmanifold air flow.

FIG. 6 shows an example method for indicating plugging of an air inletfilter based on the output of a pressure sensor positioned in thecrankcase vent tube.

FIGS. 7-8 shows example changes in crankcase vent tube pressure that maybe used to indicate a crankcase breach and identify a location of thebreach.

FIG. 9 shows an example map for indicating air filter plugging based onchanges in a crankcase vent tube pressure relative to changing manifoldair flow.

FIG. 10 shows example changes in crankcase vent tube pressure that maybe used to indicate degradation of a PCV valve.

DETAILED DESCRIPTION

The following description relates to systems and methods for monitoringcrankcase ventilation system integrity in an engine crankcaseventilation system, such as the system of FIG. 1. The output of one ormore pressure or flow sensors, such as a pressure sensor positioned in acrankcase vent tube of the crankcase ventilation system, may be used toidentify crankcase system breach, a location of the breach, PCV valvedegradation, as well as air filter plugging. An engine controller may beconfigured to perform various routines, such as the routines of FIGS.2A-B, and 3-6 to indicate crankcase ventilation system degradation basedon changes in crankcase vent tube pressure (or air flow) during enginecranking as well as changes in crankcase vent tube pressure relative tochanges in manifold air flow during engine running. The crankcase venttube pressure sensor can be orientated to read static pressure ordynamic pressure. Further, it can be placed in a venturi (necked downportion of the vent tube) and thus be sensitive to either pressure orflow rate or both. For example, the controller may determine a crankcasesystem breach based on characteristics of a transient dip in crankcasevent tube pressure, and then further identify a location and origin ofthe breach based on each of the transient dip and changes in crankcasevent tube vacuum during engine running (FIGS. 3, 4, 7, and 8). Asanother example, the controller may determine PCV valve degradationbased on deviations of an expected crankcase vent tube pressure/air flowprofile relative to an actual pressure/air flow profile (FIGS. 5, and10). Further still, the controller may detect air filter plugging (orinlet hose collapse) based on deviations of a vent tube pressure levelfrom a reference pressure during high manifold air flow conditions,wherein the reference pressure (and a related offset) is learned duringlow manifold air flow conditions (FIGS. 6 and 9). By using the samesensor to identify degradation in various system components, hardwarereduction benefits are achieved without compromising accuracy ofdetection.

Referring now to FIG. 1, it shows an example system configuration of amulti-cylinder internal combustion engine, generally depicted at 10,which may be included in a propulsion system of an automotive vehicle.Engine 10 may be controlled at least partially by a control systemincluding controller 12 and by input from a vehicle operator 130 via aninput device 132. In this example, input device 132 includes anaccelerator pedal and a pedal position sensor 134 for generating aproportional pedal position signal PP.

Engine 10 may include a lower portion of the engine block, indicatedgenerally at 26, which may include a crankcase 28 encasing a crankshaft30 with oil well 32 positioned below the crankshaft. An oil fill port 29may be disposed in crankcase 28 so that oil may be supplied to oil well32. Oil fill port 29 may include an oil cap 33 to seal oil port 29 whenthe engine is in operation. A dip stick tube 37 may also be disposed incrankcase 28 and may include a dipstick 35 for measuring a level of oilin oil well 32. In addition, crankcase 28 may include a plurality ofother orifices for servicing components in crankcase 28. These orificesin crankcase 28 may be maintained closed during engine operation so thata crankcase ventilation system (described below) may operate duringengine operation.

The upper portion of engine block 26 may include a combustion chamber(i.e., cylinder) 34. The combustion chamber 34 may include combustionchamber walls 36 with piston 38 positioned therein. Piston 38 may becoupled to crankshaft 30 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Combustion chamber34 may receive fuel from fuel injector 45 (configured herein as a directfuel injector) and intake air from intake manifold 42 which ispositioned downstream of throttle 44. The engine block 26 may alsoinclude an engine coolant temperature (ECT) sensor 46 input into anengine controller 12 (described in more detail below herein).

A throttle 44 may be disposed in the engine intake to control theairflow entering intake manifold 42 and may be preceded upstream bycompressor 50 followed by charge air cooler 52, for example. An airfilter 54 may be positioned upstream of compressor 50 and may filterfresh air entering intake passage 13. The intake air may entercombustion chamber 34 via cam-actuated intake valve system 40. Likewise,combusted exhaust gas may exit combustion chamber 34 via cam-actuatedexhaust valve system 41. In an alternate embodiment, one or more of theintake valve system and the exhaust valve system may be electricallyactuated.

Exhaust combustion gases exit the combustion chamber 34 via exhaustpassage 60 located upstream of turbine 62. An exhaust gas sensor 64 maybe disposed along exhaust passage 60 upstream of turbine 62. Turbine 62may be equipped with a wastegate (not shown) bypassing it. Sensor 64 maybe a suitable sensor for providing an indication of exhaust gas air/fuelratio such as a linear oxygen sensor or UEGO (universal or wide-rangeexhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heatedEGO), a NOx, HC, or CO sensor. Exhaust gas sensor 64 may be connectedwith controller 12.

In the example of FIG. 1, a positive crankcase ventilation (PCV) system16 is coupled to the engine intake so that gases in the crankcase may bevented in a controlled manner from the crankcase. During non-boostedconditions (when manifold pressure (MAP) is less than barometricpressure (BP)), the crankcase ventilation system 16 draws air intocrankcase 28 via a breather or crankcase vent tube 74. A first side 101of crankcase vent tube 74 may be mechanically coupled, or connected, tofresh air intake passage 13 upstream of compressor 50. In some examples,the first side 101 of crankcase ventilation tube 74 may be coupled tointake passage 13 downstream of air cleaner 54 (as shown). In otherexamples, the crankcase ventilation tube may be coupled to intakepassage 13 upstream of air cleaner 54. A second, opposite side 102 ofcrankcase ventilation tube 74 may be mechanically coupled, or connected,to crankcase 28 via an oil separator 81.

Crankcase vent tube 74 further includes a sensor 77 coupled therein forproviding an estimate about air flowing through crankcase vent tube 74(e.g., flow rate, pressure, etc.). In one embodiment, crankcase venttube sensor 77 may be a pressure sensor. When configured as a pressuresensor, sensor 77 may be an absolute pressure sensor or a gauge sensor.In an alternate embodiment, sensor 77 may be a flow sensor or flowmeter. In still another embodiment, sensor 77 may be configured as aventuri. In some embodiments, in addition to a pressure or flow sensor77, the crankcase vent tube may optionally include a venturi 75 forsensing flow there-through. In still other embodiments, pressure sensor77 may be coupled to a neck of venturi 75 to estimate a pressure dropacross the venturi. One or more additional pressure and/or flow sensorsmay be coupled to the crankcase ventilation system at alternatelocations. For example, a barometric pressure sensor (BP sensor) 57 maybe coupled to intake passage 13, upstream of air filter 54, forproviding an estimate of barometric pressure. In one example, wherecrankcase vent tube sensor 77 is configured as a gauge sensor, BP sensor57 may be used in conjunction with gauge pressure sensor 77. In someembodiments, a pressure sensor (not shown) may be coupled in intakepassage 13 downstream of air filter 54 and upstream of compressor 50 toprovide an estimate of the compressor inlet pressure (CIP). However,since crankcase vent tube pressure sensor 77 may provide an accurateestimate of a compressor inlet pressure during elevated engine air flowconditions (such as during engine run-up), the need for a dedicated CIPsensor may be reduced. Further still, a pressure sensor 59 may becoupled downstream of compressor 50 for providing an estimate of athrottle inlet pressure (TIP). Any of the above-mentioned pressuresensors may be absolute pressure sensor or gauge sensors.

PCV system 16 also vents gases out of the crankcase and into intakemanifold 42 via a conduit 76 (herein also referred to as PCV line 76).In some examples, PCV line 76 may include a one-way PCV valve 78 (thatis, a passive valve that tends to seal when flow is in the oppositedirection) to provide continual evacuation of crankcase gases frominside the crankcase 28 before connecting to the intake manifold 42. Inone embodiment, the PCV valve may vary its flow restriction in responseto the pressure drop across it (or flow rate through it). However, inother examples conduit 76 may not include a one-way PCV valve. In stillother examples, the PCV valve may be an electronically controlled valvethat is controlled by controller 12. It will be appreciated that, asused herein, PCV flow refers to the flow of gases through conduit 76from the crankcase to the intake manifold. Similarly, as used herein,PCV backflow refers to the flow of gases through conduit 76 from theintake manifold to the crankcase. PCV backflow may occur when intakemanifold pressure is higher than crankcase pressure (e.g., duringboosted engine operation). In some examples, PCV system 16 may beequipped with a check valve for preventing PCV backflow. It will beappreciated that while the depicted example shows PCV valve 78 as apassive valve, this is not meant to be limiting, and in alternateembodiments, PCV valve 78 may be an electronically controlled valve(e.g., a powertrain control module (PCM) controlled valve) wherein acontroller may command a signal to change a position of the valve froman open position (or a position of high flow) to a closed position (or aposition of low flow), or vice versa, or any position there-between.

The gases in crankcase 28 may consist of un-burned fuel, un-combustedair, and fully or partially combusted gases. Further, lubricant mist mayalso be present. As such, various oil separators may be incorporated incrankcase ventilation system 16 to reduce exiting of the oil mist fromthe crankcase through the PCV system. For example, PCV line 76 mayinclude a uni-directional oil separator 80 which filters oil from vaporsexiting crankcase 28 before they re-enter the intake manifold 42.Another oil separator 81 may be disposed in conduit 74 to remove oilfrom the stream of gases exiting the crankcases during boostedoperation. Additionally, PCV line 76 may also include a vacuum sensor 82coupled to the PCV system. In other embodiments, a MAP or manifoldvacuum (ManVac) sensor may be located in intake manifold 42.

The inventors herein have recognized that by positioning pressure sensor77 in the crankcase vent tube 74, a breach in crankcase system integritycan be detected not only at high engine air flow conditions, but also atlow engine air flow conditions based on pull-down of vacuum in the venttube. At the same time, the crankcase vent tube pressure sensor 77 canalso see crankcase pulsations. This allows crankcase system degradationto be identified more accurately while also enabling a location ofcrankcase system breach to be reliably discerned. As such, since thepressure sensor in the vent tube is used to infer or estimate thepresence of air flow through the vent tube, the pressure sensor can alsobe used as (or interchanged with) a flow meter or a gauge. Thus, in someembodiments, crankcase system breach can also be identified using a flowmeter or a venturi in the crankcase vent tube. Since flow through thecrankcase vent tube is also affected by the opening/closing of PCV valve78, the same crankcase vent tube sensor can also be advantageously usedto diagnose PCV valve degradation. Further still, since the crankcasevent tube pressure sensor will sense the compressor inlet pressureduring engine running conditions when engine air flow is elevated, theneed for a CIP sensor can be reduced. Additionally, since flow throughthe crankcase vent tube is also affected by the plugging state of airfilter 54, the same crankcase vent tube sensor can also beadvantageously used for the diagnosis of air filter clogging. In thisway, by using an existing crankcase vent tube pressure or air flowsensor of an engine system for diagnosing various engine components,such as a PCV valve, an intake air filter, as well as for crankcaseventilation system breach diagnosis, hardware and software reductionbenefits can be achieved in the engine system.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 108, input/output ports 110, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 112 in this particular example, random access memory 114,keep alive memory 116, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, including measurement ofinducted mass air flow (MAF) from mass air flow sensor 58; enginecoolant temperature (ECT) from temperature sensor 46; PCV pressure fromvacuum sensor 82; exhaust gas air/fuel ratio from exhaust gas sensor 64;crankcase vent tube pressure sensor 77, BP sensor 57, CIP sensor 58, TIPsensor 59, etc. Furthermore, controller 12 may monitor and adjust theposition of various actuators based on input received from the varioussensors. These actuators may include, for example, throttle 44, intakeand exhaust valve systems 40, 41, and PCV valve 78. Storage mediumread-only memory 112 can be programmed with computer readable datarepresenting instructions executable by processor 108 for performing themethods described below, as well as other variants that are anticipatedbut not specifically listed. Example methods and routines are describedherein with reference to FIGS. 2A-6.

In this way, the system of FIG. 1 enables various methods for diagnosingengine components coupled to a crankcase ventilation system based atleast on an estimated crankcase vent tube pressure. In one embodiment, amethod for an engine is enabled comprising, indicating crankcaseventilation system degradation based on characteristics of a transientdip in crankcase vent tube pressure, during engine cranking. In anotherembodiment, an engine method is enabled comprising, indicating alocation of crankcase ventilation system breach based on each of atransient dip in crankcase vent tube pressure during cranking and achange in crankcase vent tube pressure during steady-state engineairflow. In yet another embodiment, an engine method is enabledcomprising, during engine cranking, while manifold airflow is lower thana threshold, increasing throttle opening, and indicating crankcaseventilation system degradation based on a change in crankcase vent tubepressure following the throttle opening. In still another embodiment, amethod for an engine is enabled comprising, indicating intake air filterdegradation based on a pressure sensor in a crankcase vent tube. In afurther embodiment, a method for an engine is enabled comprising,indicating degradation of a valve coupled between a crankcase and anintake manifold based on characteristics of a transient dip in crankcasevent tube pressure, during engine cranking.

Now turning to FIGS. 2A-B, a method 200 is illustrated for indicatingdegradation of one or more engine components, including crankcaseventilation system components and intake air filters, based on changesin crankcase ventilation pressure (or air flow) during engine crankingand running. By using the same sensor to detect degradation in multipleengine components, cost and component reduction benefits are achieved.

At 202, an engine start from rest may be confirmed. For example, it maybe confirmed that the engine was completely stopped for a duration andthe engine is being started from the state of complete rest. Uponconfirmation, at 204, the engine may be started by cranking the enginewith the assistance of a starter motor. Next at 206, it may bedetermined whether the intake manifold vacuum is higher than a thresholdlevel. If not, then at 208, an actuator may be adjusted to raise theintake manifold vacuum to the threshold level. In one example, theactuator that is adjusted may be an intake throttle, wherein theadjusting includes increasing an opening of the throttle. In anotherexample, the actuator that is adjusted may be a PCV valve coupledbetween the crankcase and the intake manifold, wherein the adjustingincludes opening the PCV valve (if the valve is an on/off valve) orincreasing an opening of the PCV valve (is the valve is a duty-cyclecontrolled valve).

As such, the PCV valve may be responsive to both the pressure dropacross it and the flow rate of air through it. In particular, when it isin a low restriction position, the flow rate through the crankcase venttube (CVT) is large. In comparison, when it is in the high restrictionposition (sonically limited volume flow rate), the flow rate through theCVT is fixed (neglecting the relatively small blow-by component at highManVac). When the manifold vacuum becomes substantial enough to driveflow (e.g. 5 kPa) but not high enough to begin to cause a restriction inthe PCV valve (e.g. 25 kPa), a very high CVT flow rate occurs. This highflow rate shows up as a pressure dip in the CVT pressure sensor. Thepresence of this dip confirms proper PCV operation and lack of crankcasebreach.

Once the intake manifold vacuum is at the threshold level, from 206 or208, the routine proceeds to 210, wherein while the engine is beingcranked, and while holding the vacuum at or above the threshold vacuumlevel, a crankcase vent tube pressure (and/or air flow) is monitored.This includes monitoring an output of the crankcase vent tube pressuresensor during the engine cranking, while engine speed is below athreshold speed and before fuel is injected to any cylinder.

As such, during engine cranking, the intake manifold vacuum may be lowsuch that the position of PCV valve of the crankcase ventilation systemis open (e.g., the PCV valve may be maximally open, or at a maximumeffective area position). This causes a large flow of air to be drawnthrough the intake air cleaner, then through the crankcase vent tube,then through the crankcase, into the intake manifold. This flow throughthe crankcase vent tube towards the intake manifold can be detected by aflow meter or venturi as a transient increase in air flow at thecrankcase vent tube, or by a pressure sensor as a transient drop incrankcase vent tube pressure (or transient increase in crankcase venttube vacuum). As the engine speed increases following cranking, andmanifold vacuum increases, the air flow through the crankcase vent tubeinto the intake manifold may decrease. Thus, at 212, the routineincludes estimating characteristics of a transient dip in crankcase venttube pressure during the cranking. The characteristics estimatedinclude, for example, an amplitude of the transient dip, a timing of thedip (e.g., with respect to engine speed or piston position), a durationof the dip, etc.

Next at 214, the routine includes determining and indicating crankcaseventilation system degradation based on one or more characteristics ofthe transient dip in crankcase vent tube pressure during enginecranking. As discussed above, during engine cranking, when manifoldvacuum is lower, an increased air flow from the air filter through thecrankcase vent tube towards the intake manifold is seen as a transientdip in crankcase vent tube pressure (or transient increase in vent tubevacuum or air flow). However, this transient dip may be affected by thepresence of a crankcase system breach (e.g., if the vent tube isdisconnected), as well as the position of the PCV valve (e.g., the PCVvalve being stuck open or stuck closed). Thus, as elaborated at FIGS.3-4, crankcase ventilation system integrity breach, as well as alocation of the breach, may be indicated based at least on an amplitudeof the transient dip in crankcase vent tube pressure. For example, inresponse to the amplitude of the transient dip being smaller than athreshold during cranking, a crankcase system breach may be determined.

Following crankcase system breach detection, the routine proceeds to 216wherein PCV valve degradation is determined based on the characteristicsof the transient pressure change at the crankcase vent tube. Aselaborated at FIG. 5, this includes indicating PCV valve degradationbased on an estimated profile of the crankcase vent tube pressuredeviating from an expected profile during engine cranking. It will beappreciated that while the routine shows PCV valve degradation beingdetermined after crankcase system breach is diagnosed, in alternateembodiments, the diagnostics may be performed in parallel.

After diagnosing crankcase system breach and PCV valve degradationduring engine cranking, at 218, the routine includes injecting fuel tothe engine cylinders and initiating a first cylinder combustion event.During the engine cranking, intake manifold air flow may be lower and asthe engine speed increases (e.g., to an idling speed), the intakemanifold air flow may gradually increase. The controller may thencontinue cylinder combustion events to enable engine run-up. At 220, itmay be confirmed that the intake manifold air flow (or engine inlet airflow) is higher than a threshold air flow. As such, once the engine isat or above an idling speed, manifold air flow as well as crankcase venttube pressure may be at steady-state levels. In particular, engine speed(along with throttle position) impacts the intake manifold pump downcharacteristic during crank and run up, thereby affecting a PCV valveposition.

At 222, the routine includes monitoring the steady-state manifold airflow and the steady-state crankcase vent tube pressure. Then, at 224 and226, the routine includes determining degradation of the crankcaseventilation system and degradation of an intake air filter based on theestimated change in crankcase vent tube pressure during steady-stateconditions. As elaborated at FIGS. 3-4, this includes, at 224,indicating crankcase system degradation based on a change (e.g.,decrease) in steady-state crankcase vent tube pressure relative to achange (e.g., increase) in the steady-state manifold air flow duringengine running. As elaborated at FIG. 5, indicating air filterdegradation includes, at 226, indicating a degree of air filter pluggingbased on a rate of change (e.g., rate of decrease) in steady-statecrankcase vent tube pressure during engine running. As elaboratedtherein, the air filter plugging/hose collapse detection is performedduring engine running since the diagnostic has maximum sensitivity athigh engine air flow rates. It will be appreciated that while theroutine shows air filter degradation being determined in parallel tocrankcase system breach diagnosis, in alternate embodiments, thediagnostics may be performed sequentially.

At 228, after all the diagnostic routines have been performed, one ormore diagnostic codes may be set to indicate degradation of the affectedengine component. As such, different diagnostic codes may be set toindicate air filter plugging, crankcase system breach (includingdifferent codes to indicate the location/nature of the breach), and PCVvalve degradation. At 230, the routine includes performing anappropriate mitigating action based on the indication and the diagnosticcode that was set.

In one example, the controller may also record a number of crankcasebreach detections to determine if a threshold number of breachdetections have been reached. For example, the diagnostic routines ofFIGS. 2A-B may be rerun multiple times during a given engine operationduration including being rerun continuously from key-on until key-off,as well as during key-off. When the routine indicates a crankcasebreach, the controller may store each instance of breach detection forthat engine operation duration, and execute a notification routine oncea threshold number of detections have been reached. The threshold may beone breach detection in some embodiments. In other embodiments, to avoidfalse positive tests, the threshold may be multiple breach detections,such as two, five, ten, etc. Once the threshold number of breachdetections is reached, a message may be displayed to the vehicleoperator, such as by activating a malfunction indicator light (MIL), tonotify the operator of the vehicle of the detected crankcase breach. Inaddition, the operator may be prompted to check for possible breachlocations (e.g., a loose or missing oil cap, or by a misaligned/loosedipstick). Alternatively, the likely location of breach (as determinedat FIG. 4, elaborated below) may be indicated.

The mitigation actions may also include adjusting one or more operatingparameters to prevent additional engine damage during engine operationwith a breached crankcase, PCV valve or plugged filter. For example, themitigating actions may include acting to delay a depletion of lubricantfrom the crankcase if the crankcase is indicated to be breached. Otherexample mitigating actions include reducing an intake of air into theengine, limiting a speed or torque of the engine, limiting a fuelinjection amount supplied to the engine, limiting a throttle opening,limiting an amount of boost, disabling the turbocharger, and/or variousother actions intended to limit an aspiration of engine lubricant fromthe breached crankcase. In some embodiments, the mitigating action takenmay be one of a plurality of mitigating actions taken when a crankcasebreach is detected. As yet another example, the plurality of mitigatingactions may include adding lubricant to the crankcase or pumpinglubricant from an auxiliary reservoir and into the crankcase.

In one example, in response to a crankcase vent tube being disconnected,boosted engine operation (that is, where MAP>BP) may be limited ordiscontinued. In another example, in response to an oil cap coming offor an oil dipstick coming out of position, an engine speed may belimited, By limiting an engine speed, oils slings may be reduced sinceat high engine speeds, sling oil is more likely to exit via the oilcap/dipstick than at slow engine speeds. As still another example, inresponse to a PCV valve being stuck shut, no failure mode action may beperformed since the blow-by gas (and any entrained oil mist) is simplyrouted to the compressor inlet and then combusted. In an alternateexample, a controller may limit engine speed by a larger amount inresponse to the indication of the crankcase vent tube being disconnectedwhile limiting the engine speed by a smaller amount in response to theindication of PCV valve degradation.

Now turning to FIG. 3, a method 300 is shown for indicating crankcaseventilation system degradation based on characteristics of a transientdip in crankcase vent tube pressure during engine cranking. The methodfurther enables crankcase ventilation system degradation to bedetermined based on a change in crankcase vent tube pressure relative toa change in manifold air flow during engine running conditions.

The routine of FIG. 3 works on the principle that if the dip occurs(that is, if there is high CVT flow while a PCV valve is in a lowrestriction position) then PCV system integrity can be confirmed (withthe exception of a disconnect at first side 101). A disconnect at firstside 101 can be easily determined in vehicles equipped with a MAFsensor. For vehicles without a MAF sensors, the disconnect at first side101 is detectable by the lack of pressure drop with high engine air flowat MAF sensor 58 or CVT pressure sensor 77.

At 302, the routine includes estimating a crankcase vent tube pressureduring engine cranking and monitoring a transient dip in crankcase venttube pressure during the engine cranking. The crankcase vent tubepressure may be estimated or inferred by one of a pressure sensor, aflow sensor, or a venturi coupled in the crankcase vent tube. As usedherein, estimating crankcase vent tube pressure during the enginecranking includes before a first combustion event from rest. That is,before fuel injection to any engine cylinder. When the flow rate throughthe CVT is low, the CVT pressure sensor is effectively a static pressuresensor. It sees both the steady flow pressure drop due to flow acrossthe air cleaner and the crankcase pressure pulsations. Tube disconnectsand crankcase breaches affect the pulsation amplitude. At 304, anamplitude of the transient dip may be determined and compared relativeto a threshold amplitude. In one example, the threshold amplitude may bebased on manifold vacuum during the engine cranking. Herein, thethreshold may be increased as the expected flow through the PCV valvechanges. That is, during some condition, the threshold amplitude mayincrease with increasing manifold vacuum, and during other conditions,the threshold amplitude may decrease with increasing manifold vacuum.

If the amplitude of the transient dip is lower than the threshold, thenat 314, the routine determines and indicates crankcase ventilationsystem degradation. That is, in response to insufficient air flowthrough the crankcase vent tube during cranking, a system breach may bedetermined. Indicating crankcase ventilation system degradation includesindicating that the crankcase vent tube is disconnected. For example,the crankcase vent tube may have gotten disconnected at a first sidewhere the vent tube is mechanically coupled to the air intake passage(upstream of a compressor), or at a second, opposite side where the venttube is mechanically coupled to the engine crankcase via an oilseparator. As elaborated at FIG. 4, the controller may be configured toperform an additional routine to identify the location and nature of thebreach (e.g., location of the disconnection of the vent tube) based oneach of the transient dip in crankcase vent tube pressure during enginecranking (when an engine air flow is lower) and a change in steady-statecrankcase vent tube pressure relative to a change in steady-statemanifold air flow during engine running conditions (when the engine airflow is higher). In this way, a controller may indicate disconnection ofa crankcase vent tube from an engine crankcase ventilation system basedon changes in air flow through the crankcase vent tube during enginecranking and engine running.

Returning to 304, if the amplitude of the transient dip is not lowerthan the threshold, it may be possible that there is no crankcase systembreach. To confirm this, the routine proceeds to further determinecrankcase system breach during engine running conditions, after enginecranking. Specifically, at 306, it may be confirmed that manifold vacuumis higher than a threshold. That is, it may be confirmed that the enginehas crossed the engine cranking state and is running at or above adefined engine speed (e.g., at or above engine idling speed) when engineair flow rate (inferred of measured) is higher. Upon confirming thatmanifold air flow is higher than the threshold, at 308, the routineincludes monitoring a change in steady-state crankcase vent tubepressure relative to a change in steady-state manifold air flow. Inparticular, as the engine runs and engine speed increases, thesteady-state manifold air flow may gradually increase. At the same time,in the absence of any breach, the crankcase vent tube pressure may beexpected to gradually decrease (that is, an amount of vacuum generatedin the crankcase vent tube may increase due to increased air flowthrough the crankcase vent tube).

At 310, it may be determined if the decrease in steady-state crankcasevent tube pressure (CVT) is proportional to the increase in steady-statemanifold air flow during the engine running. That is, it may bedetermined if there is more than a threshold amount of vacuum beinggenerated at the crankcase vent tube during engine running at highengine air flow. If the change in steady-state crankcase vent tubepressure and steady-state manifold air flow during engine running isproportional, then at 312, it may be determined that there is nocrankcase ventilation system degradation, or breach. If the change isnot proportional, then the routine proceeds to 314 to indicate crankcaseventilation system degradation (e.g., that the crankcase vent tube isdisconnected) based on a decrease in crankcase vent tube pressure notbeing proportional to an increase in manifold air flow over a durationwhile the engine speed is at or above a threshold speed. For example, inresponse to reduced or no vacuum generation in the crankcase vent tubeat higher engine air flows, crankcase breach is determined. As usedherein, determining if the decrease in steady-state crankcase vent tubepressure (CVT) is proportional to the increase in steady-state manifoldair flow during the engine running may include determining if theirratio deviates from a threshold ratio, or if their absolute differenceis larger than a threshold difference.

A controller may indicate the crankcase ventilation system breach at 314by setting a diagnostic code. Further, in response to the indication,one or more mitigating actions may be performed. These may include, forexample, limiting of an engine speed and load so as to reduce/delayleaking of lubricant from the crankcase and aspiration of lubricant intoengine components. Example maps used to identify crankcase system breachare illustrated herein at FIGS. 7-8.

Now turning to FIG. 4, method 400 illustrates a routine that may beperformed to determine a location of crankcase system breach based oneach of a transient dip in crankcase vent tube pressure during crankingand a change in crankcase vent tube vacuum during and after enginerun-up.

At 402, it may be confirmed that the amplitude of the transient dip incrankcase vent tube pressure at cranking is smaller than a threshold. Aselaborated at FIG. 3, during engine cranking, when engine air flow islower, a higher air flow through the crankcase vent tube may beexperience (in the absence of a breach) which is detected by thecrankcase vent tube pressure sensor as a transient dip in vent tubepressure (or transient increase in vent tube vacuum). If there is abreach, an amplitude of the transient tube may be reduced.

Upon confirmation, at 404, it may be determined if a ratio of thedecrease in steady-state crankcase vent tube pressure (CVT) duringengine running (that is, after engine cranking while engine speed ishigher than a threshold) to the increase in steady-state manifold airflow during the engine running is lower than a threshold ratio.Alternatively, it may be determined if the absolute difference betweenthem is larger than a threshold difference. As such, it may bedetermined if vacuum generation at the vent tube during higher engineair flows is at or below a threshold level.

In still another embodiment, if a transient dip is observed, it may bedetermined that the PCV system is not degraded, and the controller maythen check for a disconnect at first side 101. This may be done bylooking for a corrupted MAF reading and a pressure drop at the MAPsensor being too small at high engine air flow rates. Alternatively, thedisconnect at the first side may be identified based on a pressure dropat the CVT pressure sensor 77 being too small at high engine air flowrates. The detection of pulsations at the CVT pressure sensor 77 mayalso be used.

In response to the transient dip in crankcase vent tube pressure duringcranking being lower than a threshold amplitude and the decrease insteady-state crankcase vent tube pressure during the increase insteady-state manifold air flow during engine running being lower than athreshold rate, at 406, crankcase ventilation system breach may bedetermined at a first side of the crankcase vent tube. For example, inresponse to a subdued transient dip in crankcase vent tube pressureduring engine cranking and substantially no crankcase vent tube vacuum(zero vacuum) generated during engine run-up, a breach is determined atthe first side of the vent tube. Specifically, it may be determined thatthe crankcase system breach is due to the crankcase vent tube beingdisconnected at a first side where it is mechanically connected to anair intake passage. Example maps used to identify crankcase systembreach at the first side are illustrated herein at FIG. 7.

In comparison, in response to the transient dip in crankcase vent tubepressure during cranking being lower than a threshold amplitude and thedecrease in steady-state crankcase vent tube pressure during theincrease in steady-state manifold air flow during engine running beinghigher than a threshold rate, at 408, crankcase ventilation systembreach may be determined at a second side of the crankcase vent tube.For example, in response to a subdued transient dip in crankcase venttube pressure during engine cranking and reduced crankcase vent tubevacuum generated during engine run-up, a breach is determined at thesecond side of the vent tube. Specifically, it may be determined thatthere is a crankcase system breach at a second, opposite side of thecrankcase vent tube where it is mechanically connected to the crankcase.As such, crankcase system breach at the second side may include one ofdisconnection of the crankcase vent tube from the crankcase at thesecond side, detachment of a crankcase oil fill port cap, detachment ofa crankcase oil level dipstick, and blockage of the crankcase vent tubeat the second side.

To distinguish between the difference crankcase system breaches at thesecond side, the routine then proceeds to 410 wherein an orifice size ofthe breach is determined. In one example, an orifice size of the breachmay also be determined. At 412, it may be determined if the orifice sizeis larger than a threshold size. If yes, then at 414, detachment of thecrankcase oil fill port may be determined based on the orifice sizebeing larger than the threshold. Else, at 416, it may be determined thatthe breach at the second side is due to disconnection of the crankcasevent tube from the crankcase at the second side, detachment of thecrankcase oil level dipstick, or blockage of the crankcase vent tube atthe second side. Example maps used to identify crankcase system breachesat the second side are illustrated herein at FIGS. 7-8.

As such, when the PCV valve is in the low restriction (fully open)position, normally large flows of air result in the crankcase vent tube.The PCV valve may be in this position due to a standard pneumaticcontrol, active PCM control, or a PCV valve fault. This high air flowrate registers as a pressure drop or a flow rate increase at thecrankcase vent tube pressure/flow rate sensor. In one example, manifoldvacuum may be computed and used to infer PCV valve position. If thecrankcase is breached (cap off, dipstick out of position, or crank casevent tube disconnect at crankcase) then the high air flow rate while PCVvalve is open does not register. For example the pressure dip does notoccur or is distinguishably reduced. The amplitude of the pressure dipor magnitude of the crankcase vent tube air flow rate also goes down asthe area (orifice area or orifice size) of the breach increases. Oil capoff and hose disconnect are likely to completely eliminate the dip. Somereduced dip may also occur for a dipstick out of position.

Upon determining a location and nature of crankcase system breach at406, 414, and 416, the routine proceeds to 418 to indicate the locationand nature of crankcase system breach by setting a diagnostic code. Assuch, a different diagnostic code may be set based on whether breach isdetected at the first side or the second side of the crankcase venttube, and further based on the nature of the breach at the second side.At 420, an MIL may be illuminated and/or a message may be set to notifythe vehicle operator about the nature and location of the crankcasesystem breach. At 422, one or more engine operating parameters may beadjusted to temporarily limit engine power so as to reduce leakage oflubricant from the breached crankcase ventilation system and aspirationof lubricant into engine components (which can degrade engineoperation).

As such, if the crankcase vent tube is disconnected at the main engineair duct (that is, at the compressor inlet, herein also referred to asthe first side) the high air flow rate during PCV valve fully open willstill be detected. In one example, in response to the indication of abreach located on the first side of the crankcase vent tube, or a breachlocated on the second side of the crankcase vent tube, an engine controlsystem may limit an engine boost. For example, boosted engine operationmay be discontinued.

Now turning to FIG. 7, an example crankcase system integrity breachdiagnostic is shown at maps 700, 710, and 720. Specifically, maps700-720 show characteristics of a transient dip in crankcase vent tube(CVT) pressure during cranking at the respective upper plots (plots 702,712, 722) and characteristics of a drop in crankcase vent tube pressurewith increasing manifold air flow during engine running (steady-stateconditions) at the respective lower plots (plots 704, 714, 724). Theupper plots of the maps are plotted over time of engine operation whilethe lower plots of the maps are plotted over engine air flow rate (asdepicted) along the x-axis.

As elaborated previously, the plumbing arrangement of the crankcase venttube as well as the specific location of the crankcase vent tubepressure sensor within the tube cause the crankcase vent tube to go to avacuum at high engine air flow rates. Thus, if the sensor detects thevacuum, it may be determined that there is no breach and that the venttube is correctly attached. However, if a vacuum is not detected, abreach in crankcase system integrity is determined. As such,disconnection of the vent tube at either side (at a first side where itis connected to the air intake passage or a second side where it isconnected to the crankcase) may result in reduced vacuum at high engineflow rates (with the degree of reduction in vacuum differing based onwhether the breach is at the first or second side). In addition, whendisconnected at the second side, crankcase pulsations may not be sensed.

Map 700 shows a first example wherein the amplitude of the transient dipin CVT pressure (plot 702) is greater than a threshold amount,indicating sufficient air flow through the vent tube during enginecranking. In addition, during engine running, a decrease in steady-stateCVT pressure is proportional to an increase in steady-state manifold airflow (plot 704). In other words, as an engine air flow increases, asmaller but gradual flow passes through the vent tube, and acorresponding vacuum is generated and sensed by a pressure or flowsensor in the crankcase vent tube.

Map 710 shows a second example wherein the amplitude of the transientdip in CVT pressure (plot 712) is smaller than the threshold amount,indicating insufficient air flow through the vent tube during enginecranking. In addition, during engine running, a decrease in steady-stateCVT pressure is not proportional to an increase in steady-state manifoldair flow, but the decrease is still more than a threshold rate (plot714). Specifically, reduced vacuum is sensed by a pressure or flowsensor in the crankcase vent tube during high engine air flow conditions(as compared to vacuum generated in the absence of a breach, as shown atplot 704). Herein, in response to the transient dip in crankcase venttube pressure during cranking being lower than the threshold amplitudeand the decrease in crankcase vent tube pressure during the steady-stateincrease in manifold airflow being higher than the threshold rate,crankcase ventilation system breach at a second side of the crankcasevent tube is indicated. The second side corresponds to a side where thecrankcase vent tube is mechanically coupled to the crankcase. Aselaborated at FIG. 8, various crankcase system breaches at the secondside can be further distinguished based on crankcase vent tube pressureand flow characteristics.

Map 720 shows a third example wherein the amplitude of the transient dipin CVT pressure (plot 722) is smaller than the threshold amount (in thedepicted example, smaller than the amplitude of plot 702 but larger thanthe amplitude of plot 712), indicating insufficient air flow through thevent tube during engine cranking. In addition, during engine running, adecrease in steady-state CVT pressure is not proportional to an increasein steady-state manifold air flow, with the decrease being less than athreshold rate (plot 724). Specifically, substantially no vacuum (zerovacuum) is sensed by a pressure or flow sensor in the crankcase venttube during high engine air flow conditions (as compared to vacuumgenerated in the absence of a breach, as shown at plot 704). Herein, inresponse to the transient dip in crankcase vent tube pressure duringcranking being lower than the threshold amplitude and the decrease incrankcase vent tube pressure during the steady-state increase inmanifold airflow being lower than the threshold rate, crankcaseventilation system breach at a first side of the crankcase vent tube isindicated. The first side corresponds to a side where the crankcase venttube is mechanically coupled to the air intake passage. For example, itmay be indicated that the breach at the first side is due to thecrankcase vent tube being disconnected from the air intake passage atthe first side.

Now turning to FIG. 8, an example crankcase system integrity breachdiagnostic is shown at maps 800, 810, and 820 for differentiatingbetween different conditions that may lead to a breach identified at thesecond side of the crankcase vent tube. Specifically, maps 800-820 showcharacteristics of a transient dip in crankcase vent tube (CVT) pressureduring cranking at the respective upper plots (plots 802, 812, 822) andcharacteristics of a drop in crankcase vent tube pressure withincreasing manifold air flow during engine running (steady-stateconditions) at the respective lower plots (plots 804, 814, 824). Allupper plots are shown over time of engine operation along the x-axiswhile all lower plots are shown over engine airflow rates along thex-axis.

Map 800 shows a first example of a crankcase system breach at the secondside of the crankcase vent tube caused by a crankcase oil fill port capcoming off. Herein, an amplitude of the transient dip in CVT pressure(plot 802) is smaller than the threshold amount, indicating insufficientair flow through the vent tube during engine cranking. In addition,during engine running, a decrease in steady-state CVT pressure is notproportional to an increase in steady-state manifold air flow.Specifically, no vacuum is sensed by a pressure or flow sensor in thecrankcase vent tube after a threshold engine air flow level (plot 804).Herein, further based on an orifice size of the breach being larger thana threshold amount, an oil cap off condition is indicated.

Map 810 shows a second example of a crankcase system breach at thesecond side of the crankcase vent tube caused by a crankcase oil leveldipstick being dislodged. Herein, an amplitude of the transient dip inCVT pressure (plot 812) is smaller than the threshold amount, indicatinginsufficient air flow through the vent tube during engine cranking. Inaddition, during engine running, a decrease in steady-state CVT pressureis not proportional to an increase in steady-state manifold air flow(plot 814). Specifically, no vacuum is sensed by a pressure or flowsensor in the crankcase vent tube during high engine air flowconditions. Herein, further based on an orifice size of the breach beingsmaller than a threshold amount, a dipstick out condition is indicated.

It will be appreciated that in embodiments where the crankcase vent tubeincludes a venturi with a coupled pressure sensor, in response to an oilcap coming off or a dipstick being out of position, a large resultingair flow through the venturi can be sensed as a deep vacuum by thecoupled pressure sensor. As such, the vacuum generated due to an oil capcoming off may be more than the vacuum generated due to the dipstickbeing out of position.

Map 820 shows a third example of a crankcase system breach at the secondside of the crankcase vent tube caused by the crankcase vent tube beingblocked or clogged at the second side. Herein, an amplitude of thetransient dip in CVT pressure (plot 822) is smaller than the thresholdamount, indicating insufficient air flow through the vent tube duringengine cranking In addition, during engine running, an increase insteady-state CVT pressure is observed during an increase in steady-statemanifold air flow. Specifically, high (positive) pressure is sensed by apressure or flow sensor in the crankcase vent tube during high engineair flow conditions. In response to these conditions, clogging of thecrankcase vent tube at the second side (coupled to the crankcase) isdetermined.

In this way, an existing sensor used for crankcase ventilation systemmonitoring can be advantageously used to also reliably identify alocation and nature of crankcase system integrity breach.

Now turning to FIG. 5, an example method 500 is shown for indicatingdegradation of a PCV valve (that is, a valve coupled in a positivecrankcase ventilation line between a crankcase and an intake manifold)based on changes in crankcase vent tube pressure and/or air flow rateduring engine cranking. As such, the routine of FIG. 5 may be performedafter confirming whether a crankcase breach has been determined based oncharacteristics of the transient dip.

As such, the method of FIG. 5 evaluates the PCV flow characteristicsduring engine running (or during a service procedure) given that boththe pressure drop across the PCV valve (manvac) and the flow ratethrough the valve (CVT flow rate) are measured by the CVT pressuresensor). In some embodiments of FIG. 5, the method may simply verify CVTflow rates at given manvacs. Therein, at the most restricted PCV valveposition, the CVT flow rate will be substantially low such that it is inthe noise. At the least restrict flow rate position, the flow rate willbe significant (that is, a transient dip will be seen).

At 502, the routine includes confirming that engine inlet air flow islower than a threshold flow. In one example, engine inlet air flow maybe lower than the threshold flow during engine cranking and early run upwhen engine speed is lower than a threshold speed and before a thresholdnumber of combustion events have occurred. Next, at 504, it may beconfirmed that manifold vacuum is lower than a threshold vacuum level.For example, it may be confirmed that manifold vacuum is less than 40kPa. If manifold vacuum is not lower than the threshold, then at 505, anactuator may be adjusted to provide a desired manifold vacuum level. Forexample, a throttle opening may be adjusted so as to hold the manifoldvacuum below the threshold vacuum level. As such, since throttle openingis related to flow rate through a PCV valve, the throttle opening may beadjusted to provide a manifold vacuum level (e.g., 13 kPa) so as toprovide maximal flow through the PCV valve.

The routine of FIG. 5 uses the output of a crankcase vent tube pressuresensor to estimate PCV valve degradation. Specifically, a gauge pressuresensor in the crankcase vent tube may be advantageously used as a flowmeter to sense changes in air flow rate in the crankcase vent tube.However, such a pressure sensor may correlate any vacuum in thecrankcase vent tube as a flow. In other words, a flow through thecrankcase vent tube may be sensed as a vacuum at the crankcase vent tubepressure sensor, and likewise, a vacuum in the crankcase vent tube mayalso be sensed as a vacuum at the crankcase vent tube pressure sensor.Thus, by performing the diagnostic routine when engine inlet air flow islower than a threshold flow, a crankcase vent tube pressure sensoroutput is relied on only during conditions when the engine inlet airflow is itself not causing a vacuum to be sensed. Likewise, byperforming the diagnostic routine when manifold vacuum is lower than athreshold vacuum level, a crankcase vent tube pressure sensor output isrelied on only during conditions when the manifold vacuum is itself notcausing a vacuum to be sensed. In addition, during conditions whenengine inlet air flow is low and manifold vacuum is low (that is, duringengine cranking and early run-up), an air flow rate through thecrankcase vent tube is expected to be high. Thus, by performing thediagnostics during those conditions, PCV valve diagnostics that arebased on changes in crankcase vent tube air flow are enabled only whenthere is sufficient air flow through the vent tube for a reliablediagnosis.

At 506, the routine includes determining an expected crankcase vent tubepressure and/or air flow profile based on current engine inlet air flowand manifold vacuum levels. The expected profiles may include anexpected vent tube pressure and expected vent tube flow rate for a givenengine speed. At 508, the routine includes estimating an actualcrankcase vent tube pressure and/or air flow profile based on the outputof the crankcase vent tube pressure sensor. It will be appreciated thatin alternate embodiments, the estimated profile may be based on theoutput of a dedicated crankcase vent tube flow sensor or a pressuresensor coupled to the neck of a crankcase vent tube venturi. Theestimated profiles may include a measured and/or inferred vent tubepressure and measured and/or inferred vent tube flow rate for the givenengine speed.

As such, during engine cranking, and the subsequent run-up, the PCVvalve is first in a more open position (e.g., at a maximally openposition when manifold vacuum is lower and throttle opening is small).During these conditions, air flow through the crankcase vent tube issubstantially higher, and can be estimated by the crankcase vent tubepressure/flow sensor as a transient increase in vent tube air flow or atransient decrease in vent tube pressure. Then, when engine speed isabove a threshold, and manifold vacuum is higher, the PCV valve may bein a second, less open position (e.g., at a smaller fixed orificeposition enabling lower flow). For example, at the second position, flowthrough the PCV valve may be controlled to a sonic choked hole. Duringthese conditions, air flow through the crankcase vent tube drops andstabilizes to a steady-state, which can also be estimated by thecrankcase vent tube pressure/flow sensor. If a PCV valve is stuck open,the crankcase vent tube air flow may continue to rise at the highermanifold vacuum conditions instead of dropping and stabilizing at thesteady-state value. Likewise, if the PCV valve is stuck in the smallorifice position during cranking, the crankcase vent tube air flow maynot rise to the expected values during the lower manifold vacuumconditions. Thus, by comparing the characteristic changes in an expectedflow/pressure profile of a crankcase vent tube pressure to the actualchanges in a crankcase vent tube flow/pressure profile as estimated by acrankcase vent tube pressure/flow sensor, PCV valve degradation can beidentified.

Accordingly, at 510, the measured or estimated crankcase vent tubepressure profile and/or air flow profile may be compared to the expectedcrankcase vent tube pressure profile and/or air flow profile and it maybe determined if an absolute difference between the profiles is largerthan a threshold. That is, it may be determined if the expected andactual crankcase vent tube pressure values or flow rates deviate fromeach other by more than a threshold amount. If not, then at 512, theroutine determines that there is no PCV valve degradation.

If there is a deviation, then at 514, it is determined that the PCVvalve may be degraded and the routine may proceed to determine thenature of the degradation based on characteristics of the estimatedcrankcase vent tube pressure and/or flow rate profiles. In particular,at 516, it may be determined if the estimated crankcase vent tubepressure or air flow rate is greater than the expected crankcase venttube pressure (or air flow rate) by more than the threshold amount.Alternatively, it may be determined if an estimated amplitude of atransient dip in crankcase vent tube pressure is higher than an expectedamplitude (or threshold amplitude). If yes, then at 518, it may bedetermined than the estimated crankcase vent tube pressure/air flowprofile is greater than the expected profile (or that the amplitude ofthe transient dip in crankcase vent tube pressure is higher than anexpected amplitude) due to the PCV valve being stuck in the openposition. The controller may indicate the same by setting an appropriatediagnostic code.

If the estimated crankcase vent tube pressure or air flow rate is notgreater than the expected crankcase vent tube pressure (or air flowrate), then it may be confirmed that the estimated crankcase vent tubepressure or air flow rate is smaller than the expected crankcase venttube pressure (or air flow rate) by more than the threshold amount.Alternatively, it may be determined if an estimated amplitude of atransient dip in crankcase vent tube pressure is lower than an expectedamplitude (or threshold amplitude). Upon confirmation, at 522, it may bedetermined if a condition of crankcase breach has already beendetermined. As previously elaborated with reference to FIGS. 2A-B, acrankcase ventilation system integrity breach may have been determinedbefore initiating the PCV valve diagnostic routine of FIG. 5. Asexplained with reference to FIGS. 3-4, a breach in crankcase ventilationsystem integrity, as well as a location of the breach may be determinedbased on characteristics of a transient dip in crankcase vent tubepressure during engine cranking, as well as a change in steady-statecrankcase vent tube pressure relative to a change in steady statemanifold air flow during engine running.

As such, if there is a breach in the crankcase system integrity, theremay be a change in one or more of the crankcase vent tube pressure andflow rate, either of which may have an effect on the crankcase vent tubepressure/flow sensor output, and resulting profile during enginecranking and run-up. In addition, the profile is affected by thelocation of the crankcase breach. For example, crankcase system breachesoccurring on the second side of the crankcase vent tube (that is, theside of the crankcase vent tube that is coupled to the crankcase) maycause the crankcase vent tube flow rate to be substantially reduced dueto the breach causing a short circuit in the expected flow rate. Inaddition, the crankcase vent tube pressure sensor may no longer show avacuum at high engine air flow rates (as compared to the vacuum shown athigh engine air flow rates in the absence of a breach). Breaches on thesecond side of the vent tube that may cause these effects include, forexample, disconnection of the vent tube from the crankcase at the secondside, a crankcase oil fill port cap coming off, or a crankcase oil leveldipstick being displaced. As another example, crankcase system breachesoccurring on the first side of the crankcase vent tube (that is, theside of the crankcase vent tube that is coupled to the air intakepassage) may cause the crankcase vent tube flow rate to be substantiallyunaffected, however, the crankcase vent tube pressure sensor may nolonger show a vacuum at high engine air flow rates (as compared to thevacuum shown at high engine air flow rates in the absence of a breach).Breaches on the first side of the vent tube that may cause these effectsinclude, for example, disconnection of the vent tube from the air intakepassage at the first side.

Accordingly, if no crankcase breach has been previously determined, at524, the routine determines that the estimated crankcase vent tubepressure/air flow profile is smaller than the expected profile (or thatthe amplitude of the transient dip in crankcase vent tube pressure issmaller than an expected amplitude) due to the PCV valve being stuck ina low flow open position (e.g., in a small orifice position or a closedposition). The controller may indicate the same by setting anappropriate diagnostic code. As such, the diagnostic code set toindicate PCV valve degradation due to the valve being stuck open (at518) may be distinct from the diagnostic code set to indicate PCV valvedegradation due to the valve being stuck closed (at 524). If crankcasebreach was previously determined, at 526, the controller may determinethat the PCV valve may be functional and not degraded.

It will be appreciated that in some embodiments, in addition toconfirming if crankcase system breach was determined at 522, it may alsobe determined if an air intake filter was diagnosed and if so, a degreeof air filter clogging may be factored into the PCV valve diagnostic. Aselaborated at FIG. 10, if air filter plugging is confirmed, then at 524,the deviation between the expected profile and the estimated profile maybe due to the air filter being clogged rather than the PCV valve beingstuck in the low flow position. The controller may distinguish betweenthese conditions based on the (known) degree of filter plugging inrelation to the observed deviation between the estimated and expectedcrankcase vent tube flow rate profiles. For example, if the deviation ismore than that expected factoring in the degree of filter plugging,crankcase system breach may be determined.

In this way, PCV valve degradation may be determined based on changes inair flow rate through a crankcase vent tube, as estimated by a crankcasevent tube pressure or flow sensor, during engine cranking. Based ondeviations of an expected flow profile from an estimated flow profile,PCV valve degradation due to a stuck open valve may be betterdistinguished from degradation due to a stuck closed valve. Byperforming the PCV valve diagnostic routine after completing a crankcasesystem breach diagnostic routine, changes in crankcase vent tubepressure or flow caused due to a crankcase system breach at either acrankcase side or an air intake passage side of the crankcase vent tubecan be factored in to enable a reliable PCV valve diagnostic. Inparticular, changes in crankcase vent tube air flow due to a crankcasesystem breach (e.g., due to a disconnected vent tube or a displace oilfill port cap) can be better distinguished from those due to a degradedPCV valve.

In one example, in response to the PCV valve being stuck open (or in thehigh flow position), an engine boost may be limited so that MAP is belowBP. As such, a stuck open PCV valve results in crankcase gasses and oilmist being blown into the inlet of the compressor. This leads to a rapidoil consumption risk which can be reduced by limiting (or discontinuing)boost. In comparison, a stuck closed PCV valve results in essentially astale air crankcase ventilation system. Over a long term, this result inengine sludge formation in the oiled portions of the engine. Thus, nomitigating action may be needed. Alternatively, in response to the PCVvalve being stuck closed (or in the low flow position), an engine speedmay be limited.

It will be appreciated that while the routine of FIG. 5 is depicted asbeing performed while an engine is cranking, in alternate embodiments,such as in embodiments where the engine is coupled in a hybrid vehiclesystem, or in engine start/stop systems where the engine is configuredto be selectively deactivated responsive to idle-stop conditions, theroutine of FIG. 5 may also be performed during key-off conditions (thatis, where a vehicle operator has turned an ignition key to an offposition). For example, during a vehicle key-off condition, a controllermay close an intake throttle and perform a vacuum decay test with thePCV valve in any given position. PCV valve degradation may then bedetermined based on the rate of vacuum decay from the crankcase venttube.

An example PCV valve diagnostic is illustrated at map 1000 of FIG. 10.Specifically, map 1000 shows changes in crankcase vent tube air flowrate along the y-axis and changes in manifold vacuum along the x-axis.Plots 1002-1008 depict example changes in vent tube flow rate relativeto manifold vacuum used for diagnosing a PCV valve.

Plot 1002 depicts a first plot of expected change in crankcase vent tubeair flow rate during engine cranking and run-up. As previouslyelaborated, during engine cranking, when manifold vacuum is low (andthrottle opening is small), the PCV valve may be in an open positioncausing a large amount of air to be directed from an intake air filter,through the crankcase vent tube, via the crankcase, into the intakemanifold. As a result, at low manifold vacuum levels (e.g., at or around13 kPa), a substantially high rate of air flow through the crankcasevent tube may be seen. Then, as the engine proceeds from cranking intorun-up, a throttle opening may increase, a PCV valve opening maydecrease (e.g., to a fixed smaller orifice position or a low flowposition), a manifold vacuum may increase (e.g., above 13 kPa), and airflow into and through the crankcase vent tube may decrease, causing adrop and eventually stabilizing of crankcase vent tube air flow rate.

Plot 1004 shows a second plot of an estimated change in crankcase venttube air flow rate during engine cranking and run-up in the presence ofa stuck open PCV valve. Herein, as the engine proceeds from crankinginto run-up, the PCV valve opening does not decrease, as expected to,due to the PCV valve being stuck open. Consequently, as the manifoldvacuum increases, air flow into and through the crankcase vent tube maycontinue to increase, causing the estimated crankcase vent tube air flowrate and profile (plot 1004) to be higher than the expected air flowrate and profile (plot 1002).

Plot 1006 shows a third plot of an estimated change in crankcase venttube air flow rate during engine cranking and run-up in the presence ofa PCV valve that is stuck in a low flow position. Herein, during enginecranking, the PCV valve may not be able to open to the fully openposition causing a substantially smaller amount of air to be directedfrom the intake air filter, through the crankcase vent tube, via thecrankcase, into the intake manifold. As a result, at low manifold vacuumlevels, a substantially smaller rate of air flow through the crankcasevent tube may be seen, causing the estimated crankcase vent tube airflow rate and profile (plot 1006) to be lower than the expected air flowrate and profile (plot 1002).

Plot 1008 shows a fourth plot of an estimated change in crankcase venttube air flow rate during engine cranking and run-up in the presence ofa functional PCV valve and an air filter that is fully clogged. Herein,as at plot 1006, during engine cranking, even though the PCV valve isopen, air flow from the intake air filter, through the crankcase venttube, via the crankcase, into the intake manifold, may be reduced due tothe clogged air filter. As a result, at low manifold vacuum levels, asubstantially smaller rate of air flow through the crankcase vent tubemay be seen, causing the estimated crankcase vent tube air flow rate andprofile (plot 1006) to be lower than the expected air flow rate andprofile (plot 1002).

In one example, plot 1002 is observed if the PCV valve is not degraded,plot 1004 is observed if the PCV valve is stuck in a low restrictionposition, plot 1006 is observed if the PCV valve is stuck in a highrestriction position, and plot 1008 is observed if the air filter isclogged or frozen shut.

It will be appreciated that while the example of FIG. 10 illustratesdetermining PCV valve degradation based on deviations in an estimatedvent tube air flow rate profile from an expected air flow rate profile,in alternate example, the same may be determined (or illustrated as)deviations in an estimated vent tube vacuum profile from an expectedvacuum profile. In this way, an existing sensor used for crankcaseventilation system monitoring can be advantageously used to alsoreliably diagnose a PCV valve.

Now turning to FIG. 6, an example method 600 is shown for indicatingdegradation of an intake air filter based on crankcase vent tubepressure estimated by a pressure sensor in the crankcase vent tube. Assuch, the routine of FIG. 6 may be performed as part of the routine ofFIGS. 2A-B.

At 602, the routine includes confirming whether manifold air flow islower than a first threshold. By confirming that manifold air flow islower than the first threshold, it may be confirmed that a sensor offsetis calculated during low engine flow conditions (such as during noengine flow) so as to reduce noise disturbances arising in thecalculation from engine flow. Next, at 604, a crankcase vent tubepressure may be estimated during the low manifold air flow conditions bya pressure sensor positioned in the crankcase vent tube. The pressuresensor in the crankcase vent tube may be, for example, an absolutepressure sensor or a gauge pressure sensor. In embodiments where thepressure sensor is an absolute pressure sensor, it may or may not becoupled to a barometric pressure sensor. In embodiments where thepressure sensor is a gauge sensor, an absolute barometric pressuresensor (e.g., BP sensor 57 of FIG. 1) may be coupled to it (e.g.,additionally present outside of the filtered volume) or used inconjunction.

At 606, the routine includes calculating a sensor offset. Specifically,the algorithm used zeroes the gauge pressure sensor during low engineflows, or learns a sensor offset based on the barometric pressurereading from the BP sensor at low engine flow conditions. In this way,the controller effectively learns or infers the barometric pressure fromthe crankcase vent tube pressure sensor and can either use the output ofthe crankcase vent tube pressure sensor at low engine flow as barometricpressure itself, or can use the output to ensure common and calibratedreference to a barometric pressure that is separately sensed. In oneexample, barometric pressure may be separately learned from a dedicatedbarometric pressure sensor coupled to the intake passage (e.g., upstreamof the air filter), or from a compressor inlet pressure sensor (CIPsensor) positioned in the intake upstream of the compressor anddownstream of the air filter. However, by using the existing crankcasevent tube pressure sensor to estimate BP, the need for a dedicated BPsensor or a CIP sensor is reduced.

In one example, the pressure sensor in the crankcase vent tube is afirst pressure sensor and the offset is determined based on a secondpressure sensor (e.g., BP sensor) coupled downstream of the air filterand upstream of the compressor. Specifically, the offset may be based onthe output of the first pressure sensor relative to the output of thesecond pressure sensor during low manifold air flow conditions. Forexample, when the first pressure sensor is an absolute pressure sensorwithout a BP sensor, the output of the first pressure sensor may be usedto infer BP. As another example, when the first pressure sensor is anabsolute pressure sensor with a BP sensor, the difference between theoutputs of the first pressure sensor and the coupled BP sensor may beused to infer BP and learn a sensor offset. As still another example,when the first pressure sensor is a gauge pressure sensor, thedifference of the first pressure sensor from a zero reading may be usedto infer BP and calculate a sensor offset.

The calculated offset may then be stored in the controller's memory as areference pressure. The stored offset may then be retrieved and appliedduring subsequent higher engine flow conditions to determine air filterplugging, as elaborated below.

Next, at 608, it may be determined if engine air flow (or any othersignal related to engine air flow rate) is higher than a secondthreshold. By confirming that engine air flow is higher than the secondthreshold, it may be confirmed that air filter plugging is estimatedduring higher engine flow conditions, when the effect of air filterplugging on crankcase vent tube pressure is greater, so as to improvedetection accuracy. If the engine air flow is not higher than the secondthreshold, the routine may wait until the desired engine air flow levelsare reached to perform the air filter plugging diagnosis. At 610, uponconfirming that manifold air flow levels are higher than the secondthreshold, it may be confirmed that the sensor offset has been updated.This may include confirming that the sensor offset that was learnedduring the lower engine flow conditions immediately preceding the higherengine flow conditions has been stored in the controller (e.g., alook-up table has been updated with the most recently learned offset).

At 612, upon confirming that the offset has been updated, the sensoroutput(s) may be adjusted based on the updated offset. This includesadjusting the output of the crankcase vent tube pressure sensor with theupdated offset. At 614, it may be determined if the deviation betweenthe adjusted sensor output and an estimated/inferred BP is higher than athreshold. In one example, the deviation may be based on the differencebetween the sensors. In another example, the deviation is based on aratio between the sensor outputs. If the difference is not higher thanthe threshold amount, then at 616, it may be determined that the airfilter is clean and is not plugged. In comparison, if the difference ishigher than the threshold amount, then at 618, air filter plugging maybe indicated. A degree of air filter plugging may be determined based onthe difference between the adjusted sensor output and BP (e.g., relativeto the threshold).

In an alternate example, a difference between the crankcase vent tubepressure reading at high air flow (which is substantially equal to CIP)and the reference pressure estimated at low air flow may be calculated.Then, a reference air filter delta pressure may be retrieved from alook-up table. The controller may then compensate the reference airfilter delta pressure for actual conditions and calculate a plug factorfrom the ratio of delta CIP over compensated reference delta pressure.That is, the controller may estimate an instantaneous air filterplugging factor based on a ratio of the difference between the crankcasevent tube pressures estimated during high and low air flow conditionsrelative to a reference air filter drop, with a correction fornon-standard temperature and pressure (STP). In one example, STPconditions include 103 kPa and 100° F. As an example, the controller mayestimate the plugging factor using the following equation:

${{{Instantaneous}\mspace{14mu} {Plugging}\mspace{14mu} {Factor}} = {\frac{{BP} - {Offset} - {{CVP}\mspace{14mu} {sensor}\mspace{14mu} {reading}}}{{F( {{airflow}\mspace{14mu} {at}\mspace{14mu} {conditions}} )}*{F( {{BP},{I\mspace{11mu} {AT}}} )}}*{F({Airflow})}}},$

wherein the plugging factor is determined in reference to standardconditions (STP).

At 620, the controller may set a diagnostic code to indicate air filterplugging. As such, the diagnostic code for indicating air filterplugging may be distinct from a diagnostic code used to indicatecrankcase ventilation system breach/degradation. The controller may alsoilluminate an MIL light notifying the vehicle operator to service theair filter. The controller may also limit engine power so as to reducecompressor over-speeding and overheating that may be caused due to theplugged air filter.

In this way, by indicating air filter degradation based on crankcasevent tube pressure, monitoring of both crankcase system integrity aswell as air filter plugging can be performed using a single sensor setalready existing in the crankcase vent tube.

An example air filter plugging diagnostic is illustrated at map 900 ofFIG. 9. Specifically, map 900 shows changes in crankcase vent tubepressure along the y-axis and changes in manifold air flow along thex-axis. Plots 902-906 depict example changes in vent tube pressurerelative to manifold air flow used for indicating a state of an intakeair filter.

During low engine air flow conditions, such as before manifold airflowis at a first threshold AF1, an offset for the crankcase vent tubepressure sensor may be learned. For example, if the crankcase vent tubepressure sensor is an absolute pressure sensor, barometric pressure maybe inferred based on the output of the crankcase vent tube pressuresensor or based on an offset between the vent tube pressure sensor and acoupled BP sensor. With reference to map 900, P1 (extended over the mapas a dashed line) reflects the reference inferred BP when the crankcasevent tube pressure sensor is an absolute pressure sensor. In analternate example, crankcase vent tube pressure sensor may be a gaugepressure sensor, wherein an offset of the pressure sensor reading from azero reading is learned such that P1 on map 900 reflects a referencecalibrated zero pressure.

During intermediate manifold air flow conditions, such as when manifoldair flow is higher than first threshold AF1 but lower than secondthreshold AF2, no offset may be learned or applied. Then, when highmanifold air flow conditions are attained, such as when manifold airflow is higher than second threshold AF2, the learned offset may beapplied to determine an air filter plugging factor.

Plot 902 shows deviations in crankcase vent tube pressure from referenceP1, as estimated by a crankcase vent tube pressure sensor, relative tochanges in manifold air flow in the absence of air filter plugging (thatis, a clean air filter). Plot 904 shows a corresponding deviation incrankcase vent tube pressure from P1, relative to manifold air flow,when the air filter is partially plugged. Plot 906 shows changes incrankcase vent tube pressure relative to manifold air flow when the airfilter is dirty and is substantially plugged. As can be seen bycomparing plots 902-906, as the plugging factor of the air filterincreases, a deviation of the pressure from the reference P1 increases.A controller may determine the degree of filter plugging based on thedegree of deviation. In this way, an existing sensor used for crankcaseventilation system monitoring can be advantageously used to alsoreliably diagnose air filter plugging.

In this way, by positioning a pressure sensor within a crankcase venttube, changes in pressure and air flow through the vent tube can bemonitored, while packaging the sensor in a cost-efficient manner. Bycorrelating the estimated changes in vent tube pressure with expectedvalues, crankcase system integrity, air filter degradation and PCV valvedegradation may be reliably indicated. By relying on characteristics ofcrankcase vent tube pressure and flow data during engine cranking aswell as engine running, breaches in the crankcase ventilation systemlocated at a side of the vent tube coupled to an air intake passage canbe better distinguished from those occurring at a side of the vent tubecoupled to a crankcase. By making adjustments to a throttle and/or PCVvalve to enhance intake manifold vacuum during engine cranking, anaccuracy of crankcase breach detection can be increased. By using thecrankcase ventilation system pressure sensor to also identify air filterplugging, as well as PCV valve degradation, the need for additionalsensors and valves for monitoring air filter degradation and PCV valvedegradation can be reduced, providing cost and complexity reductionbenefits without reducing accuracy of degradation detection. Further, anengine crankcase ventilation system can remain active during thediagnostic procedures.

It will be appreciated that the configurations and methods disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for an engine, comprising: indicating intake air filterdegradation based on a pressure sensor in a crankcase vent tube.
 2. Themethod of claim 1, wherein indicating air filter degradation includesindicating that the intake air filter is clogged.
 3. The method of claim2, wherein indicating based on a pressure sensor in the crankcase venttube includes indicating based on a change in crankcase vent tubepressure, estimated by the pressure sensor, during conditions whenmanifold air flow is higher than a threshold flow.
 4. The method ofclaim 3, further comprising, indicating a degree of air filter pluggingbased on deviation of the crankcase vent tube pressure from a referencepressure when manifold air flow is higher than the threshold flow. 5.The method of claim 4, wherein the reference pressure is estimated bythe pressure sensor in the crankcase vent tube during conditions whenmanifold air flow is lower than the threshold flow.
 6. The method ofclaim 2, wherein the pressure sensor in the crankcase vent tube is afirst pressure sensor, and the indicating is further based on a secondpressure sensor coupled downstream of the air filter and upstream of acompressor.
 7. The method of claim 6, wherein the indicating furtherbased on the second pressure sensor includes indicating based on a firstoutput of the first sensor relative to a second output of the secondsensor during conditions when manifold air flow is higher than athreshold flow.
 8. The method of claim 6, wherein the indicatingincludes, indicating air filter plugging in response to a differencebetween the first output and the second output being greater than athreshold.
 9. A method for an engine, comprising: indicating air filterplugging based on crankcase vent tube pressure relative to a barometricpressure estimate during conditions when manifold airflow is higher thana threshold.
 10. The method of claim 9, wherein the crankcase vent tubepressure is estimated by a first pressure sensor coupled in a crankcasevent tube, and wherein barometric pressure is estimated by a secondpressure sensor coupled downstream of the air filter and upstream of acompressor.
 11. The method of claim 10, wherein the indicating includes,learning an offset between outputs of the first sensor and the secondsensor during conditions when manifold airflow is lower than thethreshold; adjusting an output of the first sensor based on the learnedoffset during conditions when manifold airflow is higher than thethreshold; and indicating air filter plugging based on the adjustedoutput of the first sensor deviating from the barometric pressureestimate by more than a threshold amount.
 12. The method of claim 11,further comprising: indicating a degree of air filter plugging based onthe deviation of the adjusted output of the first sensor from thebarometric pressure estimate, the degree of air filter pluggingincreasing as the deviation increases.
 13. The method of claim 12,further comprising setting a diagnostic code to indicate air filerplugging.
 14. The method of claim 13, further comprising, limiting anengine speed responsive to the indication of air filter plugging. 15.The method of claim 10, wherein the first pressure sensor is an absolutepressure sensor or a gauge pressure sensor.
 16. An engine system,comprising: an engine including an intake passage and a crankcase; anair filter coupled in the intake passage upstream of a compressor; acrankcase vent tube mechanically connected to the intake passagedownstream of the air filter, the tube also mechanically connected tothe crankcase via an oil separator, the vent tube located external tothe engine; a sensor coupled in the crankcase vent tube for estimating avent tube air flow and/or pressure; and a control system with computerreadable instructions for, during engine cranking, while manifold airflow is lower than a threshold; learning an offset of the vent tubesensor based at least on a barometric pressure estimate; and duringengine running, while manifold air flow is higher than the threshold;correcting an output of the vent tube sensor based on the learnedoffset; and indicating filter plugging based on a deviation of estimatedcrankcase vent tube pressure from the barometric pressure estimate. 17.The system of claim 16, wherein the controller includes furtherinstructions for, indicating a degree of filter plugging based on adegree of deviation of the estimated crankcase vent tube pressure fromthe barometric pressure estimate.
 18. The system of claim 17, whereinthe vent tube pressure sensor is an absolute pressure sensor and thebarometric pressure estimate is estimated by a pressure sensor coupledin the intake passage upstream of the air filter.
 19. The system ofclaim 17, wherein the vent tube pressure sensor is a gauge pressuresensor and the barometric pressure estimate is estimated by the gaugesensor during engine cranking.
 20. The system of claim 17, wherein thecontroller includes further instructions for setting a diagnostic codeto indicate air filter plugging.